Chemiresistors are a class of chemical sensors wherein the electrical resistance of a sensing element changes in response to the presence or absence of a chemical species. Chemiresistors are attractive for use as chemical sensors as it is relatively easy to measure and record small changes in electrical resistance.
Chemiresistors and arrays of chemiresistors for determining the level of analytes in the vapour or gas phase are well established in the art. One method of realising such a chemiresistor is to mix a conducting material such as carbon black with a conducting or non-conducting polymer (Lonergan et al., Chem Mater., 1996, 8, 2298; Doleman et al, Anal. Chem., 1998, 70, 4177; Sotzing et al, Chem Mater., 2000, 12, 593). Thin films of such a matrix are then prepared with appropriate electrodes being attached to each end of the thin film. This forms the chemiresistor. The amount of carbon black and the properties of the polymers are adjusted so that a measurable resistance can be determined. When exposed to a gas or vapour that contains an amount of analyte, the analyte is adsorbed by the carbon black/polymer matrix, which subsequently swells. The swelling causes the conducting particles to move further apart leading to an increase in resistance of the chemiresistor which can be determined by standard ac or dc methods.
More recently, thin films of gold nanoparticles coated with a variety of different monolayers of molecules have been used as chemiresistors. For instance Wohtljen and Snow (Anal. Chem., 1998, 70, 2856) describe gold nanoparticle thin films prepared on interdigitated electrodes, based on 2 nm diameter gold clusters encapsulated by monolayers of 1-octanethiol. On exposure to organic vapours such as toluene in nitrogen carrier gas reversible changes in the resistance of the thin film were observed.
A variety of compounds can be used to coat the conductive nanoparticles. For instance Vossmeyer et al. (US 2003/0109056 A1) teaches the use of dendrimers to coat gold nanoparticles to produce thin film chemiresistors with particular selectivities towards different analytes such as toluene, propanol or water in the vapour phase. S. D. Evans et al (J. Mater. Chem. 2000, 8, 183) uses p-thiophenol derivatives to encapsulate the gold nanoparticles and to modify the chemiresistor behaviour. Lewis et al (US 2005/0150778 A1) teaches the use of amine functionalized polymers as coating for carbon black composites to detect volatile fatty acids.
Additionally, Lewis et al (WO 00/00808) teaches the use of arrays of nanoparticle films as chemical sensors. Baso et al (EP 1612547 A1) teaches the use of inkjet printing to produce arrays of sensors based on metallic nanoparticles.
The above disclosures in general teach methods of using such chemiresistors for detecting the presence of an analyte in the gas or vapour phase. However, the above disclosures do not teach how to use conducting nanoparticle based chemiresistors in an ionically conductive liquid phase in particular in an ionically conductive aqueous liquid phase (i.e. an electrolyte solution). In fact, Starling et al US 2004/0200722 A1 teach that “surface moisture effectively creates a bypass resistor in parallel resistance with the sensor probe. This bypass resistor typically desensitizes the performance of the sensor”. In U.S. Pat. No. 6,458,327 B1 Vossmeyer et al teaches the use of bifunctional and multifunctional ligands to produce a nanoparticle film that can be used as a chemical sensor. Vossmeyer also teaches that the simple chemiresistor structure described by Wohtljen and Snow (Anal. Chem., 1998, 70, 2856) “may however not be suitable for detection in the liquid phase due to its structure”. In order to produce a nanoparticle film structure suitable to function as a liquid phase sensor, Vossmeyer describes the use of nanoparticle films as part of a bipolar transistor structure or as part of a resonant tunnelling device. However, the production of such bipolar transistor or resonant tunnelling devices increases the complexity of the sensor device to an undesirable level compared to that of a simple chemiresistor.